The present invention relates to a data collection system for collecting data such as the consumed quantity of electricity, water or gas from a remote terminal equipment.
There have been proposed various types of data collection system for collecting at a data collection terminal the data such as the reading of a watt-hour meter, water meter or gas meter installed in each home. For instance, there has been invented and demonstrated a data collection system wherein each remote terminal equipment has a battery power source and a clock pulse generator which is activated to generate the clock pulses when a remote terminal equipment is selected and in response to the clock pulses the data is transmitted to a data collection terminal. This system is expensive because each remote terminal equipment must be provided with a battery power source. Furthermore the voltage drop of the battery power source results in misreading and therefore the battery power source must be replaced. The data collection system of the type described is therefore not satisfactory in practice in view of accuracy in reading data and maintenance. Furthermore this data collection system has another disadvantage in that each remote terminal equipment must be provided with an oscillator capable of oscillating at a predetermined common frequency with a considerably higher degree of accuracy. This means that the costs of oscillators become high and that precise adjustment of an oscillating frequency is needed in the fabrication of remote terminal equipment. Moreover because of the aging, readjustment of an oscillating frequency is required.
In some data collection systems, the electric power is supplied from a data collection terminal to each remote terminal equipment so that the provision of a battery power source may be eliminated. However, the data collection terminal and the remote terminal equipment must be interconnected with transmission lines for transmitting the power. As a result, the installation costs increase and the installation becomes complicated.
There have proposed data collection systems wherein a data collection terminal transmits clock pulses to each remote terminal equipment so that the provision of a clock pulse generator in each remote terminal equipment may be eliminated. However, the amplitude of the clock pulses tends to change due to the electrical resistance of transmission lines interconnecting between the data collection terminal and each remote terminal equipment.
There have been further proposed data collection systems in which a data collection terminal supplies each remote terminal equipment with clock pulses, from which is derived a power voltage for operating a remote terminal equipment. Since the clock pulses act as a power signal or a data signal, the clock pulses become unstable, resulting in misreading.